Signal generating system



June 16, 1964 c. R. EVANS 3,137,754

SIGNAL GENERATING SYSTEM Filed 001;. 12, 1961 United States Patent 3,137,754 SEGNAL GENERATING SYSTEM Chauncey Richard Evans, Salt Lake City, Utah, assignor to Atuk Corporation, Sait Lake City, Utah, a corporation of Utah Filed Oct. 12, 1961, Ser. No. 144,784 3 Claims. (Cl. 84-116) This invention relates to a signal generating system and, more particularly, to a system for eliminating sound board noises in electrical musical instruments.

Typical systems for producing electrically amplified tones from musical instruments may include pickup devices for converting the mechanical vibrations of toneproducing elements into electrical signals, amplifier means for amplifying such signals, and loudspeaker means for converting the amplified electrical signals into sounds. Electromechanical transducers mounted on the musical instruments have proved to be especially desirable for use aspickup devices in such systems, and the invention will be described with particular reference thereto.

In one form, a plurality of piezoelectric transducers are mounted along the bridge of a musical string instrument so that each string passes over and engages one of the transducers. When a string is vibrated, the string produces pressure variations in the transducer. 'The electrical signal generated by the transducer has the same frequency as the vibration of the string, and this signal is transmitted to an amplifier and loudspeaker to produce a sound or note corresponding to the note to which the vibrating string is tuned.

Although this arrangement is generally satisfactory, it does result in undesired amplification of certain soundboard noises. The impulse energy applied in striking or plucking the string is transmitted to the soundboard or frame of the instrument and causes the soundboard to vibrate for a very short period of time. Since all of the transducers are secured to the soundboard, they respond to this vibration, and an audible noise or thump is emitted from the loudspeaker. These objectionable noises have been found to be produced, for example, by plucking the strings of a guitar, by hammering the strings of a piano or by key-changing operations on a harp. There is also a tendency for acoustical and mechanical feedback from the loudspeaker to the frame or soundboard, which induces vibration of the transducers.

It is an object of this invention to provide a system for minimizing soundboard or frame noises in sound-producing apparatus embodying frame mounted pickups so that only the desired sounds are amplified.

It is another object of this invention to provide a system for reducing acoustical and mechanical feedback from the loudspeakers to the pickups of amplifier systems.

It is a further object of this invention to provide a system for eliminating soundboard noise components from the signals produced in electrical string instruments having electromechanical transducers mounted thereon.

In accordance with a preferred embodiment of this invention, a plurality of electromechanical transducers of the piezoelectric type are mounted on a soundboard or frame. A music string is stretched over each transducer and in engagement therewith. The transducers are arranged on the soundboard with alternating polarities so that adjacent transducers have opposite polarities. Two

3,137,754 Patented June 16, 1964 adjacent transducers are connected together as a pair and the output signal from the'pair is fed to an amplifier. When the transducers are connected in this manner, the undesirable signals produced by vibrations of the soundboard or frame are cancelled out and not amplified.

Thispreferred embodiment of the invention is illustrated diagrammatically in the accompanying drawings, in which:

FIGURE 1 is a top plan view of an electrical string instrument having electromechanical transducer pickups arranged in accordance with this invention;

FIGURE 2 is a side elevational view of the musical string instrument; and

FIGURE 3 is an enlarged perspective view, partly schematic, of the amplifier system of this invention.

All electrical musical string instruments include the basic elements shown generally in FIGURES 1 and 2. The strings are mounted on a frame 1 which may be of metal or wood. The frame 1 may form. the soundboard or sound box for the instrument. A bridge assembly 2 is mounted transversely on the frame 1, and, at the opposite end of the frame, a transverse ridge 3 is mounted parallel to thebridge assembly 2. A plurality of strings 4 extend over the ridge 3 and the bridge assembly 2 and are secured at one end in an anchor bar 5 and atthe opposite end in tensioningdevices 6. The'tension devices 6 may be, for; example, screws or pegs to which the ends of the strings 4 are attached, so that rotation of the screws or pegs 6 applies tension to the strings 4 to tune the strings to the desired frequency.

If the musical instrument is a piano, each of the strings will be tuned to a note of the scale with the first string having the frequencyof the lowest note-and each successive string having the frequency of the next higher note. A guitar has only six strings and successive strings are tuned to frequencies of notes which are several steps higher than the preceding note. The intermediate notes are then formed by depressing the strings with the fingers to reduce the length of vibration of the string. A finger board having a plurality of frets or ridges may be provided on the frame 1 to assist in forming the intermediate notes. Also, the bridge assembly 2 and anchor bar 5 may be combined.

For the purposes of describing this invention, the simplified diagrammatic arrangement ofFIGURES 1 and2 represents any musical string instrument in which the notes may be formed by vibrating a string 4. The sound of the vibrating string is amplifiedby electrical'means. A plurality of electromechanical transducers are mounted on the base 8 of the bridge assembly 2. Each of the strings 4 passes over one of the transducers 7 and tension is applied to the strings by their respective tensioning devices 6. This arrangement is shown schematically in the enlarged view of FIGURE 3.

A piezoelectric transducer may take the form of a piezoelectric body having electrically conductive terminals on two opposite faces thereof. Compression of the body produces a positive charge on one terminal and a negative charge on the other. Tension causes the charges on the terminals to be reversed. It has been heretofore proposed to mount the transducers 7 on the base 8 with all of the negative or all of the positive terminals against the base 8. The output terminals. of each transducer body are then independently connected to an amplifier circuit. When one of the strings 4 is struck or plucked, the initial force is transmitted to the base 8 causing the base 8 to vibrate briefly. The vibration of the base 8 causes the transducers 7, which are mounted on the base, to vibrate with it. During the course of this induced vibration, each transducer is subjected to acceleration forces and inertia loads which cause a signal to be generated. Due to the mass and rigidity of the base 8, its vibration is damped out quickly, but not before each of the transducers 7 has generated a signal which, when amplified, is heard as a thump. It makes little difference whether the string is struck and the soundboard is vibrated or the soundboard is struck directly. The transducers 7 themselves have mass and thus translate the movement of the frame or soundboard into electrical energy which is amplified. Attempts to isolate the transducers from the base have been unsuccessful and although the thump may be minimized it cannot be eliminated when the transducers are mounted in the conventional manner.

In accordance with the improvement of this invention, the undesirable thumping sounds are eliminated by mounting the transducers 7 so that the terminals which are adjacent to the base 8 have the opposite polarity of that of adjacent transducers. As shown in FIGURE 3, a positive terminal of one transducer and a negative terminal of the other transducer are mounted adjacent to the base 8 and compression of both of the transducers '7 produces a positive charge on the terminal adjacent the base 8 in one transducer and a negative charge on the corresponding terminal of the other transducer 7. The transducer terminals adjacent to the base 8 are connected together and fed to an amplifier 9. When the base 8 vibrates, the voltages generated by the several transducers will be approximately equal, but since the outputs are connected 180 out of phase, the signals will cancel each other out and there Will be no input to the amplifier 9.

Instead of alternately mounting the positive and negative terminals of the transducer 7 against the base 8, like terminals of all of the transducers may be mounted against the base and an electrical connection provided between the positive and negative terminals of adjacent transducers.

Another source of undesirable vibration for the base 8 is the loudspeaker 10 of the amplifier system. Sound waves emitted by the loudspeaker may set up a vibration in the base 8 and cause corresponding vibration of the transducers 7. If they have the conventional mounting arrangement, the feedback from the loudspeaker will cause an undesirable noise. The alternately polarized arrangement of FIGURE 3 reduces undesirable sounds due to acoustical or mechanical feedback from the loudspeaker.

Although the transducer arrangement of this invention cancels out soundboard noises, it does not have any effect on the musical sounds produced by the vibrating strings. Each transducer generates electrical signals corresponding to the frequency of its own vibrating string without having the signal modified by the signal from an adjacent transducer, since the strings are highly reactive loads on the transducers and base. The violent shocks of striking or plucking the strings have a different amplitude and frequency than the vibration of the string and cause the base to vibrate at its normal periods. The vibration of the base is quickly damped out, but it has a high intensity and acts on all the transducers of the system simultaneously or nearly so. Furthermore, the adjacent strings are tuned to different frequencies and the vibration of one string generally does not induce vibration in the adjacent string.

It is not necessary that there be an even number of transducers for the system to operate effectively. The transducers are connected in high impedance circuits and operate the grid circuit of an amplifier, as shown in FIG- URE 3, with very little current flowing in the circuit. Thus, the power requirements are small. Therefore, an

.13, out of phase voltage from one transducer will nearly cancel an identical voltage from a plurality of other transducers.

The reason for alternating polarity over the total range of the key board is that there is a phase shift in the base or frame and the cancellation of noise is more perfect with this arrangement. However, it works well when successive groups are alternated. For example, the successive transducers in the bridge assembly of a piano may be divided into rectangular slabs which accommodate four successive strings. Alternate ones of the transducer slabs are polarized oppositely from the intermediate slabs. This is roughly the same as having groups of four transducers alternately phased.

It is also possible to have one idle transducer on the base near a transducer with the string and alternately phase these two transducers to get cancellation of frame noise.

Although the invention has been illustrated diagrammatically as applied to a musical string instrument, it may be used also in eliminating undesirable inertia noises in other types of piezoelectric sound pickup systems, such as hydrophones. Any system which utilizes a plurality of electromechanical transducers mounted in the same body for converting pressure impulses to electrical signals would present similar problems, and the manner in which this invention may be applied to the solution of such problems will be evident from the description given above.

Still other variations and modifications of the invention will suggest themselves to persons of ordinary skill in the art. It is intended therefore that the foregoing description be considered as exemplary only and that the scope of the invention be ascertained from the following claims.

I claim:

1. A musical instrument compn'sing frame means, a plurality of independently actuatable mechanical vibrators tuned to different fundamental frequencies and each capable of producing tones composed of a fundamental and overtones, at least one end portion of each of said vibrators being fixed to said frame means, a first piezoelectric transducer mounted on said frame means adjacent a fixed end portion of a first one of said vibrators in position to be subjected to stresses imposed by such first vibrator upon actuation thereof, a second piezoelectric transducer mounted on said frame means near said first piezoelectric transducer and adjacent a fixed end portion of a second one of said vibrators in position to be subjected to stresses imposed by the second vibrator upon actuation thereof, said first and second piezoelectric transducers being sensitive to vibrations of said frame means and being electrically connected so that the electrical signals developed in such transducers as a result of vibration of said frame means are out of phase with each other, and means for amplifying the combined electrical signals from said transducers.

2. A musical instrument comprising frame means, a plurality of independently vibratable strings fixed at their ends to said frame means and being tuned to different fundamental frequencies, a first piezoelectric transducer interposed between said frame means and a first one of said strings in position to be subjected to stresses imposed by said first string upon vibration thereof, a second piezoelectric transducer interposed between said frame means and a second one of said strings in position to be subjected to stresses imposed by said second string upon vibration thereof, said first piezoelectric transducer being spaced from said second string and said second piezoelectric transducer being spaced from said first string, said first and second piezoelectric transducers being sensitive to vibrations of said frame means and being oppositely polarized so that the electrical signals developed in such transducers as a result of vibration of said frame means are out of phase with each other, and means for combining and amplifying the output electrical signals from said transducers.

3. A musical instrument comprising frame means including a sound board, a plurality of independently vibratable strings fixed at their ends to said frame means, bridge means including a plurality of piezoelectric transducers interposed between said strings and said sound board with each of said transducers being in contact with one of said strings, alternate ones of said transducers being polarized in such a direction that compressive forces applied thereto produce positive electrical signals on a terminal thereof facing said sound board and intermediate ones of said transducers being polarized in such a direction 6 that compressive forces applied thereto produce negative electrical signals on a terminal thereof facing said sound board, means for connecting said terminals together electrically, and means for amplifying the combined electrical 5 signals from said transducers.

References Cited in the file of this patent UNITED STATES PATENTS 1,915,858 Miessner June 27, 1933 10 2,622,150 Coulter et al Dec. 16, 1952 2,875,353 Cavalieri Feb. 24, 1959 

1. A MUSICAL INSTRUMENT COMPRISING FRAME MEANS, A PLURALITY OF INDEPENDENTLY ACTUATABLE MECHANICAL VIBRATORS TUNED TO DIFFERENT FUNDAMENTAL FREQUENCIES AND EACH CAPABLE OF PRODUCING TONES COMPOSED OF A FUNDAMENTAL AND OVERTONES, AT LEAST ONE END PORTION OF EACH OF SAID VIBRATORS BEING FIXED TO SAID FRAME MEANS, A FIRST PIEZOELECTRIC TRANSDUCER MOUNTED ON SAID FRAME MEANS ADJACENT A FIXED END PORTION OF A FIRST ONE OF SAID VIBRATORS IN POSITION TO BE SUBJECTED TO STRESSES IMPOSED BY SUCH FIRST VIBRATOR UPON ACTUATION THEREOF, A SECOND PIEZOELECTRIC TRANSDUCER MOUNTED ON SAID FRAME MEANS NEAR SAID FIRST PIEZOELECTRIC TRANSDUCER AND ADJACENT A FIXED END PORTION OF A SECOND ONE OF SAID VIBRATORS IN POSITION TO BE SUBJECTED TO STRESSES IMPOSED BY THE SECOND VIBRATOR UPON ACTUATION THEREOF, SAID FIRST AND SECOND PIEZOELECTRIC TRANSDUCERS BEING SENSITIVE TO VIBRATIONS OF SAID FRAME MEANS AND BEING ELECTRICALLY CONNECTED SO THAT THE ELECTRICAL SIGNALS DEVELOPED IN SUCH TRANSDUCERS AS A RESULT OF VIBRATION OF SAID FRAME MEANS ARE OUT OF PHASE WITH EACH OTHER, AND MEANS FOR AMPLIFYING THE COMBINED ELECTRICAL SIGNALS FROM SAID TRANSDUCERS. 